1. Field of the Invention
The present invention relates to a film bulk acoustic-wave resonator (FBAR) using longitudinal vibration mode along a thickness direction of a piezoelectric layer, and a method for manufacturing the FBAR.
2. Description of the Related Art
Recent wireless communication technology has accomplished rapid development, and various approaches for achieving a high-speed transmission are developing more and more. The move to higher operation frequencies of communication devices has been motivated by the need for larger and larger data rate, and in addition, there is a great demand that high frequency communication devices should be fabricated into lighter and smaller geometries. Each of such wireless communication devices encompasses a radio frequency (RF) front-end unit, which processes radio frequency signal, and a base-band (BB) unit, which processes digital signal. In the two units, the BB unit modulates and demodulates signals by digital signal processing architecture and can be easily miniaturized, since the circuitry for the BB unit can be monolithically integrated in a single LSI chip. The circuitry for the RF front-end unit, however, is difficult to be merged into a single LSI chip, since the RF front-end unit must amplify high frequency analog signals and convert frequency in a high frequency band, therefore the RF front-end unit requires a complicated configuration, which includes oscillators and many and various passive components such as filters. Generally, a surface acoustic wave (SAW) device is used as an element for RF filters and IF filters in mobile communication devices of earlier technology. However, since resonance frequency of the SAW device is inversely proportional to a spacing between comb-shape electrodes, in frequency band exceeding 1 GHz, the spacing between comb-shape electrodes becomes 1 μm or less, it is difficult to meet the demand for higher frequency of operation, and the frequency increases higher and higher, recently. Because the configuration with the SAW device is assembled by discrete components, for instance, special substrates such as lithium tantalate (LiTaO3), the RF front-end unit is not suitable for miniaturization.
For a substitute of the SAW device, as a resonator, which attracts attention recently, there is a FBAR using longitudinal vibration mode along a thickness direction of a piezoelectric layer. The FBAR is called “bulk acoustic wave” (BAW) element, etc. Because, in the FBAR, the resonance frequency is determined by acoustic wave velocity and film thickness of the piezoelectric layer, the resonance frequency is about 2 GHz at a film thickness of one to two μm and is about 5 GHz at a film thickness of 0.4 to 0.8 μm. Therefore, the FBAR facilitates high frequency operations up to several decades GHz. As the FBARs are comparatively easy to fabricate on a silicon (Si) substrate, the FBARs are advantageous for miniaturization of the wireless communication devices.
A representative structure of the FBAR of earlier technology has been disclosed in Japanese Published Unexamined Patent Application No. 2000-69594. As shown in the Japanese Published Unexamined Patent Application No. 2000-69594, the FBAR is manufactured by the following formation process sequences. First, a cavity is formed on a Si substrate by anisotropic etching, and next, a sacrificial layer such as boro-phosphate-silicate-glass (BPSG), which is easily etched by a specific etchant, is buried in the cavity at the top surface of the substrate. Afterwards, the sacrificial layer is polished flat until the top surface of the Si substrate is exposed. By the process, the sacrificial layer is embedded in the cavity at the top surface of the Si substrate and the surface of the Si substrate surrounds the perimeter of the sacrificial layer. On the sacrificial layer, a bottom electrode, a piezoelectric layer and a top electrode are deposited successively. Afterwards, an etchant supplying conduit is excavated until the etchant-supplying conduit reaches the top surface of the sacrificial layer. Then, the sacrificial layer is removed by selective etching with etchant supplied through the etchant supplying conduit, and a cavity is selectively formed under the bottom electrode. By such a sequence of formation processes, the FBAR is completed.
According to the manufacturing method of the FBAR disclosed in Japanese Published Unexamined Patent Application No. 2000-69594, the sacrificial layer is formed at the top surface of the Si substrate, and the bottom electrode is formed on the Si substrate so as to cover the sacrificial layer, and further, on the top surface of the bottom electrode, a piezoelectric layer is stacked. Then, the piezoelectric layer is delineated to occupy a given limited area, and the top electrode is delineated on the piezoelectric layer. In the sequence of formation processes, after the piezoelectric layer is delineated, a pattern of an extraction wiring of the bottom electrode is delineated by wet etching using such solutions as potassium hydroxide (KOH), tetra methyl ammonium hydroxide (TMAH), for instance, or dry etching such as reactive ion etching (RIE) method. Aluminum nitride (AlN), or alternatively zinc oxide (ZnO) is generally adopted for material of the piezoelectric layer and especially AlN is widely used because of the material behavior that facilitates the matching with the semiconductor manufacturing processes.
However, there are problems in that in either case of using AlN or ZnO, the etching rate of the piezoelectric dielectric film is low and sufficient etching selectivity of the piezoelectric dielectric film to a metallic film for the bottom electrode is hard to be achieved, when the piezoelectric dielectric film is selectively etched to expose the bottom electrode so that the extraction wiring can be connected to the bottom electrode. When the etching selectivity of the piezoelectric dielectric film to a metallic film for the bottom electrode is not sufficient, an over-etching for assuring uniformity of the etching depth decreases the film thickness of the bottom electrode in part of an element region or over the whole element region. Then, series resistance of the bottom electrode increases and contact resistance between the extraction wiring and the bottom electrode increases due to surface roughness and degeneration of the bottom electrode.
As parameters representing resonance characteristic of the FBAR, electromechanical coupling factor kt2, which is an indicator of the effectiveness with which a piezoelectric material converts electrical energy into mechanical energy, or converts mechanical energy into electrical energy and Q-value, which is a measure of the sharpness of the resonance peak in the frequency response of the system, are employed. There are two Q-values; one is a Q-value at a resonance frequency in which electrical impedance becomes a minimum and another is a Q-value at an anti-resonance resonance frequency in which the electrical impedance becoming a maximum. When a filter is implemented by a combination of resonators, a frequency bandwidth of the filter is proportional to the electromechanical coupling factor kt2, and insertion losses in the subject frequency band are inversely proportional to a quality factor defined by product of the Q-value and the electromechanical coupling factor kt2. Since the electromechanical coupling factor kt2 is a value proper to material, there is no necessity for increasing the electromechanical coupling factor kt2, if an appropriate frequency bandwidth can be realized by improving crystal purity and controlling crystal orientation to a polarization direction. Therefore, the Q-value must be set as high as possible so as to decrease the insertion losses.
Elastic loss of the piezoelectric layer, elastic loss of the electrode, and series resistance of the electrode affect the Q-value at resonance frequency and the elastic losses of the piezoelectric layer, the elastic losses of the electrode, conductance of the substrate, and dielectric losses of the piezoelectric layer affect the Q-value at anti-resonance frequency. According to analysis of experimental data of inventors of the present invention, series resistance of the bottom electrode play a dominant role as the origin of degradation of the Q-value at resonance frequency, and elastic losses of the piezoelectric layer is the dominant origin of degradation of the Q-value at anti-resonance frequency. From the investigation, an increase of series resistance of the bottom electrode by etching failure, causes the degradation of the Q-value at resonance frequency, which greatly affects performances of the FBAR. In addition, e investigation proved a possibility in which disconnection of the bottom electrode is caused by over-etching.
So to overcome the above-mentioned problems due to over-etching, various methodologies such as:
(a) selecting material of the bottom electrode so that etching selectivity of the piezoelectric dielectric film to a metallic film for the bottom electrode is sufficiently large (hereinafter called “the first methodology”);
(b) providing some margins for over-etching by increasing the film thickness of the bottom electrode (hereinafter called “the second methodology”); and
(c) decreasing etching rate of the piezoelectric dielectric film so as to facilitate detecting the end point of the etching (hereinafter called “the third methodology”), are adopted.
However, in the first methodology, if a high selectivity is required as one of the specific material properties so as to overcome the problems associated with the low etching selectivity, freedom for selecting materials becomes small, because the material property must satisfy the required low resistance value and low elastic losses (internal friction), etc. Because the thickness of the bottom electrode has great effect upon the resonance characteristic itself, there is an optimum film thickness of the bottom electrode. If the thickness of the bottom electrode cannot satisfy the optimum film thickness, the electromechanical coupling factor kt2, which is an index to the intensity of the piezoelectricity, decreases, and further the Q-value, which is a measure of the sharpness of resonance peak in the frequency response of the system, decreases, and still further shift of the resonant frequency is generated. Therefore, the second methodology such that providing some margins for the etching process, by increasing the film thickness of the bottom electrode so as to overcome problems associated with over-etching of the bottom electrode has a limitation. Further, by the third methodology, decreasing the etching rate of the piezoelectric dielectric film so as to facilitate detecting the end point of the etching process, the processing time for each etching process becomes long, which increases the throughput time.
In view of these situations, it is an object of the present invention to provide a FBAR, which has a large electromechanical coupling factor kt2 and a large Q-value, and a method for manufacturing the FBAR.